A known sample tube holder used in clinical chemistry analyzer comprises a rectilinear array of chambers, each of which is adapted for receiving and holding in place a sample tube. All chambers of the array have the same shape and dimensions. The length symmetry axis of all chambers of the sample tube holder lie in one and the same plane, e.g. in a symmetry plane of the rectilinear array of chambers of the sample tube holder.
Primary sample tubes used in clinical chemistry analyzers usually have different diameters and usually also different lengths. In order to hold such primary sample tubes in place, each of the chambers of a known sample tube holder includes an elastic tongue which extends from the inner surface of a side wall of the chamber with an inclination towards the center thereof and which presses each sample tube against a side wall of the chamber. When several sample tubes having different diameters are inserted in respective chambers of such a sample tube holder, the length symmetry axis of the sample tubes are not aligned and lie at different distances from the plane which contains the length symmetry axis of the chambers of the sample tube holder. Sample tubes having a diameter close to the inner diameter of the chambers of the sample tube holder where they are inserted are approximately centered in those chambers, whereas the centers of smaller sample tubes clearly lie out of the centers of the chambers where they are inserted.
In another known sample tube holder the side walls of each of the chambers of the sample tube holder include four elastic tongues with an angular spacing of 90 degrees between them and these tongues press the sample tube towards the length symmetry axis of the chamber. Although centering of the sample tube can be in principle achieved in this way, in practice however manufacturing tolerances and non-uniform deformation of the tongues with time makes it impossible to ensure that each of the sample tubes inserted in the chambers of the known sample tube holder is accurately centered and that the length symmetry axis of all sample tubes inserted in the sample tube holder are accurately aligned and lie in the symmetry plane of the sample tube holder.
For the reasons indicated above, the prior art sample tube holders described above are not suitable for use in an analyzer where it is necessary that every one of the sample tubes is accurately centered in a chamber of the sample tube holder and that the length symmetry axis of all sample tubes inserted in the sample tube holder are accurately aligned and lie in the symmetry plane of the sample tube holder. This is for instance required if the sample tube holder is used in an analyzer having an automatic pipetting unit which moves the pipetting needle only along a rectilinear path which lies in the symmetry plane of the array of chambers of the sample tube holder. An accurate centering of every one of the sample tubes is also required to ensure that they are properly gripped by the gripper of a robotic device used for transporting the sample tubes.